1. Field of the Invention
This invention relates generally to an apparatus for generating an image from ultrasonic waves.
2. Description of the Prior Art
Ultrasonic systems of the type herein contemplated are disclosed, for instance, in U.S. Pat. No. 3,967,066, in Acoustical Holography, vol. 5, pages 493-503, 1974, and in Acoustical Holography, vol 6, pages 1-13.
The U.S. Pat. No. 3,971,962 discloses a linear transducer array for ultrasonic image conversion in an ultrasonic orthographic imaging system (C-scan camera). This prior art transducer array contains a large number of elongated transducer elements. The patent mentions that, from the standpoint of resolution, it would be favorable to design each element of the sampling array to be small and to have equal height and width. In other words, each element should be small in both dimensions. However, there are some problems associated with a small element: the electrical impedance of such an element is very high. This would lead to impedance matching problems in the electric circuits which detect and process the signals derived from the individual elements. This is true, for instance, for the preamplifiers which are connected to each respective element. Poor impedance matching can result in a low signal-to-noise ratio. The high impedance also leads to poor high frequency response due to the shunting effect of the inevitable stray capacitances associated with the element mounting and lead attachments to the elements.
In order to avoid these problems, elongated receiver elements are used in the prior art design disclosed in the above-mentioned patent. Each of the elongated elements corresponds or is equivalent to many small elements which are connected in parallel. A parallel connection of elements has comparatively low impedance. Thus, the impedance matching and high frequency loss problems have been solved. However, simultaneously the resolving power of the array of elements has been reduced in one dimension, that is the dimension of the longitudinal axis of the element. In order to correct this reduction of resolution, the patent suggests employing a cylinder lens which is arranged a short distance in front of the array of elongated elements. The cylinder lens is situated in a position to cause the converging wavefronts from an image-forming lens to collimate in one dimension.
It has turned out that such a cylinder lens may produce undesirable internal reverberations of the ultrasonic waves between the front and back surface of the cylinder lens. Therefore, spurious acoustic waves may be superimposed in the image field received by the elements. The superposition of these waves results in additional patterns superimposed in the true ultrasonic image which is to be displayed. It is highly desirable to avoid the superposition of such patterns.
Application of a cylinder lens may also have another effect. There may occur reflections between the elongated transducer elements and the cylinder lens. The surface of the elements has an impedance which is somewhat different from the impedance of the fluid which is conventionally interposed between the elongated elements and the cylinder lens. Thus, there may occur reflections on the surface of the elements and reflections on the surface of the cylinder lens. Again, this effect will result in undesired patterns in the ultrasonic image.
The cylinder lens itself constitutes an additional complex component, which requires some expeditures. For proper operation, the cylinder lens should be covered by a matching layer. Applying this layer requires some work and is time consuming. Therefore, it is desirable to use elongated low-impedance transducer elements, but to eliminate the otherwise concomitant requirement or necessity of a cylinder lens.